One known distress beacon location system is the MEOSAR (Middle Earth Orbit Search and Rescue) system, which is a satellite system in middle earth orbit used for search and rescue. This comprises:                signal-transmitting beacons to be located;        relay satellites in middle earth orbit (which can be satellites used in a geolocation and navigation system, a system that is often designated by the acronym GNSS, standing for Global Navigation Satellite System);        independent processing modules on the ground, or stations, sometimes also qualified as MEOLUT (Middle Earth Orbit Local User Terminals);        a centre for coordinating the processing modules on the ground, sometimes qualified as MTCF, standing for MEOLUT Tracking Coordination Facility.        
The centre for coordinating the processing modules on the ground makes it possible to assist in the programming of the antennas of the different modules in order to improve the coverage and the overall efficiency of the network of modules, in particular when the modules are equipped with a number of antennas (as is the case with some MEOLUT stations).
An SAR (Search and Rescue) distress beacon is seen simultaneously by a number of satellites which pick up the signals that it transmits and retransmit them to stations on the ground. These stations on the ground demodulate the beacon messages coded in the signals, then measure the times of arrival (TOA) of the signals as well as the frequency shift that these signals (FOA, standing for Frequency of Arrival) have undergone by Doppler effect in order to determine the position of the beacon.
However, the signal-to-noise ratio of the received signals is not always sufficient to enable the stations to analyse them in order to locate the beacon, or even in order to determine the content of its message. Thus, even when a sufficient number of satellites—generally, four satellites—has been able to transmit beacon signals, the TOA and FOA cannot be determined for all these signals, which prevents the position of the beacon from being determined.
The degradation of the signal-to-noise ratio can be inherently heterogeneous, and caused notably by the environment of the beacon during transmission, by the atmospheric propagation, by the relay satellite used, and by the environment of the MEOLUT station on reception.